1. Field of the Invention
This invention relates generally to a testing apparatus and method and more specifically to an apparatus and method for testing flexible membrane material used as seal fingers on air cushion vehicles.
2. Description of the Prior Art
Ships which travel across water supported by a bubble of air, such as air cushion vehicles (hovercraft) and surface effect ships, generally have a flexible structure for containing the air bubble on which it rides. One conventional arrangement utilizes an inflated bag which receives the air from the lift fan and seal fingers which descend from the bag and are in contact with the water. The seal fingers are made from flexible membrane material which flexes in reaction to waves on the water surface. This flexing eventually causes the failure of the material and the seal fingers must periodically be replaced.
The complex motions which these fingers undergo and the relation between failure and the various parameters of this motion are of great interest to designers of such ships. Unfortunately, there has been no testing apparatus capable of realistically reproducing the flexural motion of the seal finger so that parameters may be measured under laboratory conditions. This lack of information has hindered the development of better designs of seal fingers and development of operational standards for ships to avoid damage.
One previous attempt to test the material included horizontally attaching one end of a strip of material to a solid base with air blowing underneath the strip. The air causes the strip to flap in a manner similar to a flag, causing a flexing of the material. A second similar test uses a strip mounted in a similar fashion but with the second end bent and fastened to a weight. Air and water flowing along the inside bend of the strip causes a flexing and vibrating motion of the strip.
Other attempts at testing include shooting jets of water either continuously or in pulses at pieces of material. In one version the material is placed over a frame to achieve the same shape as in a ship and with air filling the finger from behind. All of these test procedures measure the endurance of the material due to a particular type of bending. Unfortunately, the forces involved and the type of bending performed bear no resemblance to the forces and bending encountered in actual performance. The shape of the finger itself, the drag forces encountered as the finger touches the water and the force and frequency of waves are not considered or tested at all in these procedures.